1. Field of the Invention
The present invention relates to an ultrafine multifilamentary Nb3Al superconducting wire improved in characteristics by adding Ge or Si, and provides a process for producing an ultrafine multifilamentary superconducting Nb/Al compound wire into which Ge or Si is added.
2. Description of the Related Art
It has long been known that Nb3(Al,Ge) or Nb3(Al,Si) produced by arc melting yield a superconducting critical temperature TC and a superconducting critical magnetic field HC2 far higher than those of Nb3Al.
In utilizing a production method as shown schematically in FIG. 13, recently proposed is a production process comprising subjecting a composite wire material of Nb and Al to a rapid hating and quenching treatment to thereby form a solid solution phase supersaturated with a bcc Nb-25 at. % Al alloy (the term xe2x80x9cat. %xe2x80x9d as referred herein signifies xe2x80x9c% by atomicxe2x80x9d) in the composite wire material, and then subjecting the resulting product to heat treatment in the temperature range of from 650 to 900xc2x0 C. In this manner, it is possible to deposition superfine crystals of Nb3Al precipitates near to stoichiometric composition. Since the resulting product enables an extremely high critical current density JC, this method is attracting attention in producing practically useful wire material.
Since the highest magnetic field generated by using a metallic superconducting wire material on record is 21.7 T, the upper limit attainable in generated magnetic field using a practical Nb3Al wire material is presumably about 23.5 T.
An oxide superconducting wire material enables the generation of a higher magnetic field, however, it disadvantageously requires a production cost about 100 times as large as necessary for producing a metallic superconducting wire material.
Concerning the method for producing a metallic superconducting wire material, on the other hand, there is known a production method comprising preparing an ultrafine multifilamentary composite wire comprising an Alxe2x80x94Ge alloy core embedded in a Nb matrix, and subjecting the wire to a rapid heating and quenching treatment comprising rapidly heating the wire to a temperature of ca. 2,000xc2x0 C. by resistive heating and continuously introducing it to a molten metal. In this manner, provided that the addition of Ge into Al is as small as 2% or less, a supersaturated solid solution is generated in the resulting composite wire, and by applying a post precipitation heat treatment thereto, the resulting product enables a high JC, but there is no considerable increase in TC and HC2.
On the other hand, if the amount of added Ge increases, the supersaturated solid solution becomes unstable, and the production of a supersaturated solid solution becomes available only under super quenching conditions.
However, the production process above is practically unfeasible because continuous super quenching is extremely difficult in an industrial production.
In the light of the aforementioned circumstances, in accordance with an aspect the present invention, there is provided a process for producing an ultrafine multifilamentary superconducting Nb3(Al,Ge) wire comprising: preparing a composite core material comprising an Alxe2x80x94(2-30)at. % Ge alloy (where at. % represents % by atomic) 1 xcexcm or less in thickness uniformly incorporated into a Nb matrix at a volume ratio in a range of 1:2.5 to 1:3.5 and forming a composite therewith; fabricating a composite wire having an ultrafine multifilamentary structure by embedding a plurality of the resulting composite core materials in a cylindrical matrix material containing Nb; forming a A15-phase filament having a lower order in crystallinity inside the composite wire having the ultrafine multifilamentary structure by a rapid heating and quenching treatment comprising rapidly heating the composite wire having the ultrafine multifilamentary structure to a temperature of 1,700xc2x0 C. or higher in 2 seconds, followed by continuously introducing it into a molten metal; coating the composite wire having the ultrafine multifilamentary structure in the state above with copper (Cu) which functions as a superconductivity stabilizing material; and applying a post heat treatment in the temperature range of from 650 to 900xc2x0 C. to the resulting product to recover the degree of crystallinity of the Nb3(Al,Ge) in the A15 phase compound (claim 1).
According to another aspect of the present invention, there is provided a process for producing an ultrafine multifilamentary superconducting Nb3(Al,Si) wire, comprising the same process steps as claimed in claim 1, except for using an Alxe2x80x94(2-20)at. % Si alloy (where at. % represents % by atomic) as the starting material in the place of the Alxe2x80x94(2-30)at. % Ge alloy (claim 2).
In accordance with still other aspects of the present invention, there are provided processes for producing an ultrafine multifilamentary superconducting Nb3(Al,Ge) wire or Nb3(Al,Si) wire as claimed in claim 1 or claim 2, wherein instead of coating the composite wire with copper (Cu) prior to the additional heat treatment, the step of Cu coating for stabilizing superconductivity is performed after the additional heat treatment (claim 3); a process for producing an ultrafine multifilamentary superconducting Nb3(Al,Ge) wire or Nb3(Al,Si) wire as claimed in claim 1 or claim 2, wherein copper (Cu) is surrounded beforehand with a diffusion barrier material and then embedded into the matrix material, followed by wire drawing to fabricate the composite wire having the ultrafine multifilamentary structure, and subjecting the resulting composite wire to the rapid heating and quenching treatment (claim 4); or a process for producing an ultrafine multifilamentary superconducting Nb3(Al,Ge) wire or Nb3(Al,Si) wire as claimed in one of claims 1, 2, 3, or 4, wherein an alloy expressed by Alxe2x80x94(2-30) at. % Gexe2x80x94(0-7)at. % X or Alxe2x80x94(2 -20) at. % Sixe2x80x94(0-7)at. % X (where at. % represents % by atomic), where X represents at least one element selected from the group consisting of Mg, Zn, Li, Ag, Cu, and B, is used as the starting material in the place of the Alxe2x80x94(2-30)at. % Ge alloy or the Alxe2x80x94(2-20)at. % Si alloy (claim 5).
The present invention has been accomplished based on the following findings of the present invention.
On carrying out the rapid heating and quenching treatment on a conventional Alxe2x80x94Ge core material embedded in Nb matrix as described above, filaments of A15 phase compounds having a low degree of ordering in crystallinity are formed, but by subjecting them to a heat treatment in the temperature range of from 650 to 900xc2x0 C., the long range ordering is recovered to yield a TC of 19.4 K and a HC2 (4.2 K) of 39.5 T. However, this product yields a JC(4.2 K) of 130 A/mm2 at 15 T, a value somewhat inferior as compared with that of a practical wire material. Still, a decrease in JC with increasing magnetic field for the material above of the product is small, and the JC(4.2 K) under a magnetic field of 25 T is about 100 A/mm2, i.e., the highest among the metallic superconducting wire materials. However, a practical wire was still unfeasible because a JC(4.2 K) of at least 150 A/mm2 was necessary for use as a practical superconducting magnet at the targeted magnetic field.
The present inventors successfully fabricated a composite ultrafine multifilamentary wire comprising an Alxe2x80x94Ge alloy core reduced in diameter from the conventional 1.5 xcexcm to 0.3 xcexcm, and, on applying a rapid heating and quenching treatment to the composite wire, a JC(4.2 K) of over 250 A/mm2 was finally attained under 21 T, and a value of 150 A/mm2 was achieved at 25 T.
From the fact above, it was understood that, presumably, by optimally designing a superconducting magnet using the Nb3(Al,Ge) ultrafine multifilamentary wire material above, it is possible to generate a magnetic field of 25 T under the operation at 4.2 K and 27 T under the operation at 1.8 K (that is, cooling from 4.2 K to 1.8 K improves the superconductivity by about 2 T).
In the case of adding Si to Al, the TC and HC2(4.2 K) were similarly improved. Although there was confronted with a problem of low JC in this case due to the rapid heating and quenching treatment which led to the formation of an unstable supersaturated solid solution and of an A15 phase compound having a low degree of crystallinity, an increase in JC was achieved by similarly reducing the diameter of the Alxe2x80x94Si alloy core.
The invention disclosed in this application has been accomplished based on the findings above, and it proposes a novel fabrication process for obtaining high JC in an ultrafine multifilamentary Nb3Al wire improved in TC and HC2 by adding Ge or Si.
The superconducting wire material according to the present invention fabricated by the production process described in the present application enables a generation of an intense magnetic field in the level of 27 T.